You'll be setting yourself up with some headroom for when you are ready to upgrade that CPU.... BUT
with no other changes, you are not going to notice a difference. (although, I do believe you will see some benchmarkable (is that a word?)
gains in your AGP performance.)
I suggest putting that money aside and waiting until you are also ready to upgrade the proc. The MoBo will only
get cheaper (and perhaps better with revision, etc....)
-Sid